This invention relates to a press pack semiconductor device having an improved intermediate electrode attached to a semiconductor element.
The press pack semiconductor device such as the Giant Transistor or Gate Turn Off Thyristor (respectively shortened to G-Tr or GTO in the following) has been applied for use as a high power device which has current ratings of more than 100 amperes. The press pack semiconductor device would be destroyed by high current if current concentration were to occur therein when turned on or when turned off. Accordingly, the semiconductor element of the press pack semiconductor device generally is formed in a mesa structure to protect the semiconductor element from current concentration. The mesa is so formed in an emitter region of the G-Tr element in a predetermined pattern as to divide the current flowing in the semiconductor device. The whole surface of an emitter electrode layer formed on the mesa-top of the silicon pellet is substantially short-circuited by a thick intermediate plate for decreasing thermal stress between the semiconductor element and a first disc. The thick intermediate plate is selected from a material such a molybdenum whose coefficient of thermal expansion is close to that of the silicon pellet and is electroconductive and then is formed in the plate construction to short-circuit the whole surface of the emitter electrode layer. A cathode region of the semiconductor element fixed on a supporting plate is connected to a second disc. A housing is disposed to surround the semiconductor element which is pressed by both the first disc and the second disc.
Since the thickness of the intermediate plate of the prior art semiconductor device is larger than half of the ditch width of the mesa, the cathode electrode layer of, for example, aluminum on the mesa top will be fixed solidly to the thick intermediate plate in an alloy at a relatively low temperature by heat originated near the contact area between the thick intermediate plate and the cathode electrode layer when the semiconductor device is activated at high power if the contact area is small and is subject to a high pressure. Great stress in the thick intermediate plate rigidly attached to the silicon pellet will change the crystal structure of the silicon pellet to cause a deteriorated characteristic of the semiconductor device and will rupture the semiconductor element because the molybdenum-aluminum alloy has a different coefficient of thermal expansion than silicon. In consequence, there is a need for a new press pack semiconductor device which can prevent current concentration.